1. Field of the invention
This invention relates to the field of detecting vehicles or other metallic objects by the positioning of an inductive loop on a travelled road or other surface wherein the inductive characteristics of the loop are varied when the metallic body of a vehicle or other metallic object moves into the magnetic field.
2. Description of the prior art
Inductive loop type vehicle presence detectors are known which employ detector means such as a phase detector to sense a signal which is passed through a phase shifting circuit including an inductive loop when a vehicle moves into the field of the loop. A change in the phase angle occurs upon the entry of the vehicle into the magnetic field of the inductive loop. Other presence detectors are known wherein an inductive loop is connected as a part of a tuned circuit for controlling the frequency of an oscillator. In such a system when a vehicle enters the field of the loop, the inductance value of the loop will vary to change the frequency of the oscillator. A change in the oscillator frequency is sensed to provide an output signal in a system such as this. A common failing of most of the prior art systems is the failure to sense the presence over a long period of time of a vehicle in the area of the loop.
Midlock in U.S. Pat. No. 3,571,789 discloses a vehicle presence loop detector for sensing the presence of a vehicle substantially without time limitation so long as the vehicle remains substantially within the confines of the loop. In the system disclosed by Midlock, it is apparent that the vehicle must first enter the area of the loop in order to activate the system to sense the exiting of a vehicle from the area of the loop. In addition, such a detector is disabled by a power failure after which the presence of the vehicle or object is lost and must be reestablished, upon restoration of power, by re-entering the loop.
Another vehicle detection system for detecting the presence of a vehicle which utilizes a change in impedence of a tuned circuit and corresponding phase shift as a metallic object moves into the magnetic field of the inductive loop is Marcinkiewicz, U.S. Pat. No. 3,500,310. In this system, the sudden change of voltage which occurs as a metallic object moves into the proximity of the inductive loop is stored in a storage element and simultaneously the discharge circuit for the storage element which is present in such a system is disabled. Such a system is capable of detecting the presence of a vehicle in a detection zone for a long period of time extending over many hours but is limited by the leakage of the capacitance means utilized as a storage element so that detection of a vehicle or other metallic object in the detection zone is not possible for an indefinite period of time.
It is an object of this invention to provide an improved vehicle detector that is capable of operating in either of two modes, namely a mode which detects the entry of a vehicle or other metallic object into a detection zone and a second mode in which the exiting of a vehicle or other metallic object from a detection zone is determined over an indefinite period of time. The loop detection system of the present invention is particularly suited for use as a security alarm system since the detector system of the invention can be tuned with a vehicle in the presence of the loop or off the loop. In addition, said detector system in the event of electrical power outage will maintain the initial static tuned condition (presence or absence of vehicle or object) upon restoration of power.
Other loop detector systems employing phase detectors as a means of detecting the presence of a vehicle in the vicinity of the inductive loop are U.S. Pat. No. 3,900,829; U.S. Pat. No. 3,587,040; and U.S. Pat. No. 3,870,990.